The agency needs details on more cases and more blood, tissue and urine samples from affected pets, according to an update posted Tuesday. The FDA has sent letters to U.S. veterinarians outlining the needed testing.

FDA will cover the cost of the tests to get to the bottom of the outbreak, which has mostly affected dogs, the agency says. Just 10 of the cases reported so far have been in cats.

What the pets have in common: They became ill, usually within hours, after consuming treats sold as jerky tenders or strips. The treats are made of chicken, duck, sweet potatoes or dried fruit. Typical symptoms include decreased appetite and activity, vomiting, diarrhea, increased water consumption and increased urination. Some affected pets suffer kidney failure.

In the update, FDA official Bernadette Dunham called the wave of pet illnesses "one of the most elusive and mysterious outbreaks we've encountered."

Dunham directs the FDA's Center for Veterinary Medicine, which says it has already conducted 1,200 tests and visited jerky manufacturers in China.

The agency is not issuing a recall or naming brands of jerky treats. It says most have been made in China, but notes that manufacturers are not required to list the country of origin for ingredients on pet food labels.

FDA says several jerky pet treats were removed from the market in January after testing found they contained "up to six drugs." The agency says it's unlikely the drugs caused the illnesses, but that the rate of illnesses dropped after that, probably because fewer of the products were available.

The update says consumers should "be cautious about providing jerky treats" to pets. "If you do provide them and your pet becomes sick, stop the treats immediately, consider seeing your veterinarian, and save any remaining treats and the packaging for possible testing."

More information for consumers is in an FDA fact sheet. The fact sheet says consumers can help the investigation by reporting complaints about treats through www.safetyreporting.hhs.gov or contacting pet food companies.